Although the countless lakes and woods in Sweden are its most important environmental asset, the wood and timber industry – and the paper industry in particular – have severely contaminated the lake beds with unmonitored discharges and dumping. Unacceptable levels of mercury were found in two lakes near the city of Hultsfred in Sweden.
DEME designed and dimensioned an environmental dredging method to clear the mercury sediments and other pollutants from the two lakes, even building a special new vessel called the ‘Pixie’ that is able to dredge mechanically and transport the sediments hydraulically. In the summer of 2006, we eventually dredged 280,000 m³ of contaminated sediment using the new vessel. Then after dredging the sediment was pumped to a disposal site.
This pioneering project also made large-scale use of geotubes for dewatering the sediments. By deploying a system of geotubes, the sludge is encapsulated and protected from the surrounding soil. The geotubes had a diameter of a few metres and were 50 to 100 m long. Water that came from the dewatering process was purified before discharge. A custom-built water treatment plant, with a capacity of 200 m³ per hour, treated the water over a 1.5 year period.